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Northern Rockies Skies for May

April 27, 2012 — A monthly look at the night skies of the northern Rocky
Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay
Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

To orient yourself to our May skies, the Big Dipper will be
directly overhead at dusk.

The cup of the dipper is opened northward; the dipper's
handle starts a great arc southward in the sky, arriving at the bright orangish
star Arcturus, and then ending at the bluish gem, Spica. This process sometimes
is called "arcing to Arcturus."

Arcturus, the "guardian of the bear," is the
brightest star in the kite-shaped constellation Bootes, the herdsman or
"bear driver." Spica is the brightest star in the constellation
Virgo.

Near the western horizon, you
will see the last of Regulus, the brightest star in the constellation Leo the
Lion, and the Gemini twins, Castor and Pollux. Late in the early evening,
rising above the eastern horizon, you will start to see the celestial beckoning
of summer, the three bright stars -- Vega, Deneb and
Altair -- of the "summer triangle."

In the early morning of May 4-5, the Earth passes through a
stream of cometary debris believed to once be a part of Halley's cometary
nucleus. Looking
above the eastern horizon toward the constellation Aquarius, the Aquarid meteor showers can best be seen in the early morning of May 5.

On May 20, an annular solar eclipse will pass in the Pacific
and western United States. The northern Rockies will only see a partial solar
eclipse.

For you planetary viewers: Saturn is in Virgo near Spica,
Mars is in Leo near Regulus, and Venus, the brightest of the planets, is in
Auriga, centered between Capella, Aldeberan, and the Gemini Twins, Castor and
Pollux.

The mathematician and astronomer Eratosthenes (circa 276
B.C. - 195 B.C.) was born in Cyrene, a Greek colony near the coast of present-day northeastern Libya. Eratosthenes came to prominence -- having been
educated in Alexandria -- while working for the ruler of Egypt, Ptolemy III.

He was appointed librarian of the Great Library of
Alexandria (one of the largest of the ancient world, containing several hundred
thousand scrolls, which is comparable to almost 100,000 books). Thus Eratosthenes lived
at a major center of culture and learning, and contributed several unique
advances to science. He introduced the word "geography" - the study
of the physical characteristics of the Earth's surface -- and invented the
system of latitude and longitude for terrestrial measurements.

Using measurements of the angle of the sun's rays with
respect to the vertical in both Alexandria and Syene (modern Aswan) and from
knowledge of the distance between the two cities, Eratosthenes was the first to
calculate the circumference of Earth, which he found to be 250,000
"stadia." Assuming the unit was Egyptian stadia (about 1/10 of a mile),
his measurement translates to about 25,000 miles, close to the modern-day value
of the polar circumference (24,860 miles).

From lunar eclipse data, he estimated the sun's distance to
be 804,000,000 stadia (80.4 million miles, compared to the actual value of 92.9
million miles), and the lunar distance as 780,000 stadia (low by about a factor
of three from the true value of 238,855 miles). He derived a value of about 24
degrees for the angle between the Earth's orbital plane (the ecliptic) and its
rotational axis (current value, 23.44 degrees).

Eratosthenes also introduced a calendar with leap years, and
purportedly compiled a star catalogue containing 675 stars. The "Sieve of
Eratosthenes" is a method for separating prime and composite numbers. A
prominent crater on the moon is named in his honor.